This invention relates to a line array of colinear slot radiators and, more particularly, to an array of plural parallel columns of slot radiators with excitation and phasing of electromagnetic waves controlled by a set of fin-shaped vanes upstanding from a common broad wall of a waveguide or cavity.
An array of slot radiators disposed in a staggered line along a wall of a waveguide is employed frequently to generate a beam of electromagnetic power. As a typical example of an array antenna composed of slot radiators, the antenna comprises a waveguide of rectangular cross section wherein the width of a broad wall is double the height of a narrow wall, and wherein the slots are formed through one of the broad walls. Antennas are constructed also of a plurality of these slotted waveguides arranged side-by-side to provide a two-dimensional array of slot radiators arranged in rows and columns. To facilitate description of the antenna, a column of slot radiators is considered to be oriented in the longitudinal direction, i.e., in the direction of propagation of electromagnetic power in the waveguides, and a row of slot radiators is considered to be transverse to the direction of propagation in the waveguides. An antenna composed of a single waveguide generates a fan beam while an antenna composed of a plurality of the waveguides arranged side by side produces a beam having well-defined directivity in both the plane parallel to the columns and the orthogonal plane parallel to the rows.
Antennas employing slot radiators may have slots which are angled relative to a center line of the broad wall of the waveguide, or may have slots which are arranged parallel to the center line of the broad wall of the waveguide but offset from said center line alternately on one side and the other side. In order to attain a desired linear polarization, and a desired illumination function of the radiating aperture of the entire antenna, the configuration of the antenna of primary interest herein is to be configured with all of the slots being parallel to each other and arranged colinearly in parallel columns. The colinearity eliminates unwanted grating lobes or second order beams.
A cophasal relationship among the radiations from the various slot radiators is employed for generating a broadside beam directed perpendicularly to a plane containing the plurality of waveguides. Herein, the antenna comprising the two-dimensional array of rows and columns of radiators is of primary interest. One method of obtaining the cophasal relationship is to position the slot radiators with a spacing of one guide wavelength. However, such a spacing is sufficiently large to introduce grating lobes to the directivity pattern of the antenna and, accordingly, it is preferred frequently to employ a smaller spacing, typically one half of the guide wavelength, between successive ones of the slot radiators.
However, the spacing of one half guide wavelength introduces a problem because a wave propagating along the waveguide undergoes a phase shift of 180 degrees during propagation through a distance of one-half guide wavelength. Therefore, the requirement of a cophasal relationship is contradicted by the desire to space the radiators at a distance of one-half guide wavelength. Typically, a cophasal result is obtained, despite the half guide wavelength spacing, by alternating the direction of the slot positioning used to achieve slot coupling to the energy in the waveguide.
Also, to facilitate manufacture of the antenna, and to reduce the overall weight of the antenna, it would be preferable to construct the antenna of a single waveguide having broad walls of sufficient width to form multiple columns of slot radiators within a single broad wall. This would eliminate the need for constructing multiple individual waveguides. However, such a constriction of multiple columns of slot radiators within a single broad wall introduces a further problem, namely, that consecutive slot radiators within any row of the array would be excited with radiation which differs in phase by 180 degrees. Thus, the cophasal relationship would not be attained.